The present invention generally relates to antennas adapted to operate in multiple frequency bands, and more particularly to multi band antennas used for wireless communication systems such as cellular telephone systems.
Wireless communication systems, e. g. cellular telephone systems, are usually based on radio frequency (RF) waves. There are analog and digital standards in use in various regions of the world being created to provide an acceptable level of compatibility for wireless communication systems, i.e. to standardize design criteria for cellular telephone devices. The standards differ from each other significantly in their operating frequency ranges. For example, GSM (Global System for Mobile communication) is a digital standard that typically operates at a low frequency band, such as between 880 MHz and 960 MHz, while AMPS (Advanced Mobile Phone System) is an analog standard that typically operates at frequency bands between 824 MHz and 894 MHz. Further digital standards in wide use are DCS (Digital Communication System) having high frequency bands between 1710 MHz and 1880 MHz, and PCS (Personal Communication System) having operating frequencies of 1850 MHz to 1990 MHz, wherein both DCS and PCS are based on GSM.
Cellular telephone devices used for wireless communication systems necessarily include an antenna for receiving and transmitting radio frequency signals such as the widely employed monopole antennas. Since the resonating frequency of an antenna depends on the length of the antenna in a known manner due to the wavelength of the RF-waves, a certain antenna can be used only for a certain frequency range. Due to the different standards, however, it is desirable that one and the same cellular telephone device is able to operate within widely separated frequency bands in order to utilize more than one standard. Therefore, an antenna adapted to operate in multiple frequency bands is needed.
Furthermore, cellular telephone devices are increasingly undergoing a miniaturization to cater to consumer""s demand for convenience. As a result, antennas utilized by such devices also have to become smaller and lighter. However, as antennas become smaller, the frequency bands within which they can operate typically become narrower. Consequently, helix antennas are often employed for cellular telephone devices operating within multiple frequency bands. Helix antennas typically include a conducting member wound in a helical pattern. As the radiating element of a helix antenna is wound about an axis, the axial length of the helix antenna can be considerably less than the length of a comparable monopole antenna. Hence, helix antennas can be used where the length of a monopole antenna is too extended.
Accordingly, there is a need for an antenna to be relatively compact in size and to be capable of operating in multiple widely separated frequency bands such as GSM and PCS. Small multi band antennas providing adequate bandwidth in at least two frequency bands are known.
For instance, U.S. Pat. No. 6,075,488 to Hope discloses a broadband antenna that includes a centrally positioned high frequency-radiating element surrounded by a dielectric support element, and a linear radiating element in the form of a wire wound over the dielectric support element and extending generally over the entire length of centrally positioned high frequency radiating element, thus defining an over-wound helical coil. The length of the linear radiating element is such that it supports resonance at a lower frequency. Such an antenna can resonate at two broadly separated frequencies and, therefore, is configured for dual frequency band operation.
Furthermore, U.S. Pat. No. 6,127,979 to Zhou et al. reveals a multi band antenna that comprises a fixed whip antenna element and a helical coil antenna element coupled to a single feed point. The antenna is reduced in size by attaching a disc to the end of the whip antenna element, while decreasing the pitch of the helical coil antenna element. A dielectric material surrounds the whip antenna element and provides support for the helical coil antenna element.
It turned out that a certain precisely predetermined distance between the top of the whip antenna element and the corresponding end of the helical coil antenna element is extremely important with regard to the performance of the antenna. However, mounting of the helical coil antenna element often causes variations to this distance. Such variations are difficult to control due to the elasticity of the helical coil antenna element. In addition, because of low production costs, the helical coil antenna element is usually manufactured with wide tolerances. Thus, inaccuracies in the pitch of the helical coil antenna element occur.
The present invention provides an antenna, which is adapted to operate in multiple frequency bands, and consists of a first antenna element for receiving and transmitting signals in a first frequency band, and a second antenna element for receiving and transmitting signals in a second frequency band. The first antenna element has a substantially elongated conductor of a predetermined first pitch, which is coupled to a feed point. The second antenna element has a substantially meandering coil of a predetermined second pitch, which is coupled to said feed point. The substantially meandering coil comprises a plurality of first portions having a wound form and surrounding at least partly said first antenna element, and also comprises a plurality of second portions having a straight form. The first portion extends from an upper end of said second portion to a lower end of a subsequent second portion following in axial direction of the second antenna element.
In accordance with a preferred embodiment of the present invention, the second portions can extend parallel to the elongated conductor of the first antenna element. Also, the first portions can extend concentric to the elongated conductor of the first antenna element. Preferably, the first portions and the second portions can be perpendicular.
Further in accordance with a preferred embodiment of the present invention, one first portion can be arranged at the top and another first portion can be arranged at the bottom of the second antenna element. In addition, the first portion arranged at the bottom of said second antenna element can be formed as a closed ring.
Still further in accordance with a preferred embodiment of the present invention, the first antenna element can be a fixed whip antenna. Preferably, the first antenna element can have a disk on the top of the substantially elongated conductor.
According to another embodiment of the present invention, the antenna can comprise a RF matching network that matches the first antenna element and the second antenna element.
In accordance with a preferred embodiment of the present invention, the antenna comprises a dielectric material surrounding the first antenna element, wherein the substantially meandering coil of the second antenna element is supported by the dielectric material.
Preferably, the second antenna element can be made of a punched-out metal sheet or of a formed plastic that is plated with copper. Moreover, in accordance with a preferred embodiment of the present invention, the second frequency band is one of a GSM and AMPS band and the first frequency band is one of a DCS and PCS band.
Antennas according to the present invention are particularly well suited for operation within wireless communication systems such as cellular telephone systems utilizing multiple, widely separated frequency bands. Furthermore, because of their small size, antennas according to the present invention can be employed within very small communications devices. Besides, because the second antenna element having the form of a substantially meandering coil comprises a plurality of first portions having a wound form and a plurality of second portions having a straight form, wherein the first portion extends from an upper end of said second portion to a lower end of a subsequent second portion following in axial direction of the second antenna element, antennas according to the present invention can be manufactured with a steady performance. The reason for this is that the second antenna element according to the present invention is more rigid and can be manufactured more accurately in the pitch than conventional helix antenna elements. As a result, the distance between the top of the first high frequency antenna element and the corresponding end of the second low frequency antenna element is easy to keep constant.